I've been wondering recently about why PSUs have fans. Some PSUs (like the one here: http://www.newegg.com/Product/Product.aspx?Item=N82E16817151122) don't, and they're only marginally more efficient than the ones that do. At 50% of the maximum power draw, a platinum PSU is only 7% more efficient than a bronze PSU. In the case of the unit I linked to, that's equivalent to 18.2 W. What this really means is that a bronze, fanned PSU must use forced convection to dissipate 18.2 W more than the platinum, fanless PSU dissipates with only natural convection.
I find it difficult to believe that a bronze unit couldn't operate at the slightly (only slightly) higher temperature resulting from the additional 18.2 W is must dissipate due to increased inefficiency (relative to the platinum unit). Either the platinum PSU is intended to operate at a higher temperature, and is built with higher quality components that can tolerate elevated temperatures for extended periods of time, or fanned, bronze (and higher) units don't really need fans at all. I've noted that the fanless units usually are better vented, which allows for more natural convection; if this is the only difference, though, then it would be pretty easy to use better venting on less efficient units too.
One other note: this is all approximate, because in a case, there would be a fair amount of forced convection due to other fans in the case, so really no PSU needs to dissipate heat using only natural convection.
I suppose I'm looking for comments you might have. I don't think there's anything wrong with the analysis, but if there is, please let me know.